French utility certificate No. 78 27749 describes an electronic tension reducer followed by two amplifier circuits: one of the amplifier circuits having a relatively long time constant, of the order of 100 ms, and serving to feed the measuring units, which units are not required to have a short restoration time in the event of the tension disappearing. The other amplification circuit has a much shorter time constant, of the order of 10 ms, since it is used to feed protection units that are required to have a short restoration time. Such an apparatus therefore has two measurement systems, and an object of the present invention is to provide a circuit which includes only one measurement output. Such an apparatus is clearly cheaper than the prior art.